Method of manufacture of potassium phosphate

ABSTRACT

THERE IS DISCLOSED A PROCESS FOR THE PRODUCTION OF POTASSIUM DIHYDROGEN PHOSPHATE BY THE REACTION OF POTASSIUM C HLORIDE WITH PHOSPHORIC ACID WHICH HAS PREFERABLY BEEN CONCENTRATED AND DEHYDRATED AT ELEVATED TEMPERATURES, TO FORM A REACTION MIXTURE CONTAINING POTASSIUM PHOSPHATE COMPOUNDS, HYDROLYZING THE RESULTING REACTION MIXTURE WITH WATER, OR WITH AQUEOUS PHOSPHORIC ACID AND PRECIPITATING THE RESULTANT POTASSIUM DIHYDROGEN PHOSPHATE PRODUCT BY THE ADDITION OF AN ORGANIC LIQUID.

Oct. 23, 1973 WQRTHINGTQN ETAL 3,767,770

METHOD OF MANUFACTURE OF POTASSIUM PHOSPHATE Filed Dec. '7, 1971 r i AUnited States Patent 3,767,770 METHOD OF MANUFACTURE OF POTASSIUMPHOSPHATE Ralph Eric Worthington, William Henry Thompson, and ThomasNoel Enda Somers, Dublin, Ireland, and Erhart K. Drechsel, Houston,Tex.; said Worthington, Thompson and Somers assignors to Fitzwilton,Limited, Dub: lin, Ireland, and said Drechsel assignor to PennzollCompany, Shreveport, La.

Filed Dec. 7, 1971, Ser. No. 205,613 Int. Cl. C01b 15/16, 25/26 US. Cl.423309 7 Claims ABSTRACT OF THE DISCLOSURE There is disclosed a processfor the production of potassium dihydrogen phosphate by the reaction ofpotassium chloride with phosphoric acid which has preferably beenconcentrated and dehydrated at elevated temperatures, to form a reactionmixture containing potassium phosphate compounds, hydrolyzing theresulting reaction mixture with water, or with aqueous phosphoric acidand precipitating the resultant potassium dihydrogen phosphate productby the addition of an organic liquid.

This invention relates to the production of potassium dihydrogenphosphate which is particularly valuable in presenting potassium andphosphorus in a convenient solid form which can be used either as afertilizer directly or in combination with other components.

According to the present invention this compound is obtained byphosphoric acid or polymerized phosphoric acid attack on potassiumchloride.

This invention provides a process for the production of potassiumdihydrogen phosphate comprising the steps of reacting phosphoric acidwhich has preferably been concentrated and dehydrated with potassiumchloride at elevated temperatures. During the reaction, hydrogenchloride and some water vapor are evolved and can be collected and usedas a marketable product. This reaction is generally conducted at atemperature of about 180- 325 C. with a most preferred operatingtemperature range of 250-300 C.

In the reaction the potassium chloride or potash is usually employed asa solid although use of molten potassium chloride through a preheatingstep would mitigate heat exchange requirements. The phosphoric acidreactant is preferably a concentrated phosphoric acid having a P 0content of about 60-80%, preferably 7075%, i.e., superphosphoric acid.

In the reaction it has been found that the removal of the hydrogenchloride is a relatively easy matter if the reaction is carried out inthe presence of an excess of acid but it is not so easily achieved ifstoichiometric quantities are used. Thus, it is a preferred feature ofthe invention to use excess phosphoric acid. As indicated,superphosphoric acid is preferably employed. However, 50 called wetprocess phosphoric acid can also be employed. Irrespective of the acidused, it should be employed in such amount as to provide a P:K ratio inthe reaction of above 1.8:1 and preferably 1.8:1 to 5:1. A highlypreferred excess of acid to be used is the amount necessary to provide aP:K ratio of 1.9:1 to 2.5:1 in the reaction. The ratios expressed aremolar ratios.

Although the separation of the potassium dihydrogen phosphate from theacid liquor may be achieved by crystallization this is not always easy.The preferred method for the separation of potassium dihydrogenphosphate is by the addition of a low boiling point water solubleorganic liquid such as an alcohol, ketone or soluble ether. Typicalorganic liquids which may be used include those "ice which are misciblewith Water such as the lower alkyl alcohols including methanol, ethanol,isopropanol, etc. as well as lower alkyl ketones such a acetone, methylethyl ketone, etc. as well as mixtures thereof. Methanol is highlypreferred because of the good results provided and its readyavailability. The liquid is usually employed in a weight ratio of liquidto total solution of about 0.5 :1 to 3:1, preferably 1:1. The treatmentis usually carried out with agitation at a temperature of about 20-60 C.The treatment of the phosphoric acid solution with the organic liquidprovides a means whereby substantially all of the potassium dihydrogenphosphate can be caused to precipitate by addition of the organic liquidwith substantially all of the phosphoric acid remaining in the solution.

On treatment with the organic liquid the potassium phosphate isprecipitated and the acid liquor is then stripped of the organic liquidin a simple distillation column and returned to the process or separatedby other means. In order to avoid the precipitation of the solids as asviscous intractable mass it has been found necessary to hydrolyze thecomplex phosphates in the reaction mass before precipitation. Hydrolysisis carried out by heating with water. The water may be supplied by wetprocess phosphoric acid which can be one of the raw materials fed to theprocess. The wet process acid which contains about 20% water is fed tothe hydrolyzer and an economy in the operation of the systems results. Asignificant feature of the hydrolysis step of the process when practicedin a continuous manner is that the source of the water can also be theaqueous phosphoric acid solution fed to the process. Therefore in thismanner the hydrolysis can be effected by a feed material of the process.Obviously however, other sources of water for the hydrolysis may beused.

The solids precipitated by the organic liquid are washed either on theseparator or by repulping in the organic liquid with subsequentseparation. The crystals are dried and sent to storage.

The invention is illustrated by the accompanying drawing which is aflow-sheet of one embodiment of the process.

The apparatus comprises a reactor 1 with a potassium chloride feeder 2,a hydrolyzer 4 with a phosphoric acid feeder 3, which are followed by aprecipitator 6, a separator and washer 7, a dryer 8, a distillationcolumn 11, an evaporator 14 and an organic liquid feeder 12 which is fedfrom a supply 13.

In the operation of the process potassium chloride is fed from feeder 2and phosphoric acid recycled from the process fed at 15 to the reactor 1where hydrogen chloride evolved is removed at 5. The mixture ofpotassium phosphate and phosphoric acid is fed to a hydrolyzer 4together with wet process phosphoric acid or water from a feeder 3 andfrom thence to a recrystallizer 6 together with an organic liquid forexample, methanol, from feeder 12 and washings from the washer 7B. Theresulting suspension is fed to the separator 7A and the crystals areseparated and washed in the washer 7B. The product is finally dried inthe dryer 8 to form a product 9. Heat is supplied at 10. The liquor fromthe separator 7A containing phosphoric acid residual potassium phosphateand the organic liquid is fed to a distillation column 11 where theorganic liquid is recovered and the phosphoric acid and potassiumphosphate is concentrated in the evaporator 14 and water is removed at17. The resulting solution is fed back to the reactor at 15 as shown oralternatively it may be used for the manufacture of pure phosphate anddrawn off at 16.

A particularly advantageous feature of the invention is its readyadaptability to a continuous operation. A major portion of thecontinuous operation is shown in the drawing in the aspects ofcontinuous reuse of the organic solvent with recovery in column 11,storage in organic liquid supply 13 for use via organic liquid feeder12. Also the phosphoric acid solution recovered from distillation column11 may be evaporated in evaporator 14 to the de sired concentration andrecycled for reaction with the potash. Moreover, a portion of thisrecovered phosphoric acid solution may also be withdrawn for themanufacture of other phosphates. Accordingly, the process is one inwhich substantial economies in operation are achieved because of theeasy adaptation to continuous operation.

The invention is exemplified by the following working example 'but it isnot to be considered as limited thereto.

EXAMPLE Potassium chloride and purified recycle phosphoric acid (70.3% Pwere reacted together continuously at a temperature of 250 C. in suchproportions as to give a P to K mole ratio in the reactor of 2:1. Theliquid residence time was 2 /4 hours. The chloride level in the reactorproduct was less than 0.1%

Wet process phosphoric acid (50% P 0 was added and the mixture heated at100 C. for 1 hour. The resulting mixture, called hydrolyzed liquor had aP to K mole ratio of 4: 1.

Impure potassium phosphate was precipitated by the addition of methanolto the hydrolyzed liquor in the ratio of 0.75 to 1 on a weight basis.The solids were filtered, washed with methanol and removed from thesystem. The acid filtrate was fed to a distillation column and themethanol removed. The resulting bottoms solution containing 56.1% P 0was evaporated to yield superphosphoric acid (70.3% P 0 One third of theevaporated acid was removed from the system as superphosphoric acidsubstantially free of all impurities except potassium. The remainingsuperphosphoric acid was fed back to the reactor wtih potassiumchloride. The impure potassium phosphate contained 46% P 0 and 31% K 0.

The invention has been described with reference to certain preferredembodiments. However as obvious variations thereon will become apparentto those skilled in the thereto.

What is claimed is:

1. A process for the production of potassium dihydrogen phosphatecomprising:

(1) reacting potassium chloride with concentrated phosphoric acid having2. P 0 content of about 60- 80% wherein the amount of phosphoric acidreacted is an sufi'icient excess over stoichiometric quantities toprovide a P:K molar ratio in the reaction of about 1.8:1 to 5:1, andwherein the reaction is conducted by contacting the reactants at atemperature of about 180325 C. with hydrogen chloride evolution to forma reaction mixture containing complex potassium phosphate compounds;

(2) contacting the resultant reaction mixture with water or mixturescontaining water to hydrolyze any complex phosphate present in themixture;

(3) contacting the hydrolyzed mixture with a low boiling point watersoluble organic liquid selected from the group of lower alkyl alcohols,lower alkyl ketones, or mixtures thereof in a weight ratio of liquid tohydrolyzed mixture of about 0.511 to 3:1 to precipitate the potassiumdihydrogen phosphate as a solid and provide a solution of phosphoricacid in the organic liquid;

(4) removing the potassium dihydrogen phosphate as a solid; and

(5) removing the organic liquid and recovering the phosphoric acid.

2. A process according to claim 1 wherein the concentrated phosphoricacid reactant has a P 0 content of about 707 5% and the reaction isconducted at a temperature range of 250-300 C.

3. A process according to claim 2 wherein a sufficient excess ofphosphoric acid is used to provide a P to K molar ratio of 1.9: 1 to2.5: 1.

4. A process according to claim 3 wherein the treatment with the organicliquid is conducted at a temperature of about 2060 C.

5. A process according to claim 4 wherein the hydrolysis step isconducted by contacting the resultant reaction mixture with a portion ofthe aqueous phosphoric acid fed to step (1).

6. A continuous process for the production of potassium dihydrogenphosphate which comprises the steps of:

(1) continuously reacting potassium chloride with concentratedphosphoric acid having a P 0 content of about 60-80% wherein the amountof phosphoric acid reacted is in suflicient excess over stoichiometricquantities to provide a P:K molar ratio in the reaction of about 1.821to 5:1, and wherein the reaction is conducted by contacting thereactants at a temperature of about -325 C. with hydrogen chlorideevolution to form a reaction mixture containing complex potassiumphosphate compounds;

(2) contacting the resultant reaction mixture with a portion of aqueousphosphoric acid to hydrolyze complex phosphates present in the mixture;

(3) contacting the hydrolyzed mixture with a low boiling pointwater-soluble organic liquid selected from the group of lower alkylalcohols, lower alkyl ketones or mixtures thereof in a Weight ratio ofliquid to hydrolyzed mixture of about 0.5 :1 to 3:1 to precipitate thepotassium dihydrogen phosphate as a solid and provide a solution ofphosphoric acid in the organic liquid;

(4) removing the potassium dihydrogen phosphate as a solid; and

(5) removing the organic liquid and recycling to the precipitation step(3) and recovering the phosphoric acid.

7. A continuous process according to claim 6 wherein in step (1), thephosphoric acid has a P 0 content of 70-75% and the reaction isconducted at a temperature of about 250-300 C. employing sufiicientexcess phosphoric acid to provide a P to K molar ratio of 1.9:1 to 2.5:1, in step (3) the precipitation step is conducted at a temperature ofabout 2060 C. and the recovered phosphoric acid is concentrated andrecycled as feed for further reaction.

References Cited UNITED STATES PATENTS 2,902,341 9/1959 Baniel et a123l07 X 2,894,813 7/1959 Baniel et a1 23107 X 1,805,873 3/1931 Kaselitz23l07 1,865,968 7/1932 Schuppe 23l07 2,824,786 2/ 1958 Medub-Sobet 23l07FOREIGN PATENTS 4,112,729 7/1966 Japan 23l07 739,119 11/1969 Belgium23l07 1,102,711 3/1961 Germany 23l07 1,272,905 7/ 1968 Germany 23l07EDWARD J. MEROS, Primary Examiner G. A. HELLER, Assistant Examiner US.Cl. X.R. 71-34, 43

